The present invention relates to seat belt retractors for spooling seat belts (webbings) for retaining passengers in seats of vehicles and the like. More specifically, it relates to a seat belt retractor which can transmit large torque and is provided with a torsion bar which can be easily formed by forging.
A seat belt device provided in a vehicle, such as an automobile, restrains a passenger from an abrupt movement caused by acceleration due to collision of the vehicle, thereby ensuring safety of the passenger. The seat belt device generally includes a webbing or belt, a retractor, a buckle, and the like. The retractor retracts the webbing by winding the webbing about a spool using a spring force. The retractor then locks the webbing from being pulled out from the spool when applied with an impact, thereby restraining the passenger. The buckle serves for making the webbing to fit the passenger""s body, and is generally disposed at a side of the seat.
Conventionally, in the seat belt device, the webbing has been locked from being pulled out from the retractor in a collision, and the passenger""s body has been restrained from moving forward by the locked webbing. However, a load would be applied to the passenger at the breast, etc., via the webbing, when the passenger is rapidly restrained from moving forward. It has been confirmed that the load applied to the passenger can be effectively reduced by drawing out the webbing by a given length while applying a predetermined resistance to the draw-out movement. That is, when restraining the passenger from moving, the webbing is preferably drawn out by a given length while being applied with a predetermined load in the opposite direction, thereby absorbing energy of collision applied to the passenger.
The structure for absorbing energy applied to the passenger is called an energy absorption xe2x80x9cEAxe2x80x9d structure.
A seat belt retractor which uses a torsion bar as the EA structure is known. In this type of seat belt retractor, a spool for spooling a webbing is non-rotatably connected to a relatively fine torsion bar at one end of the torsion bar. A locking structure to prevent the webbing from pulling out is connected to the other end of the torsion bar. Whereby, the rotation of the spool may be locked via the torsion bar. The force required to restrain the passenger from moving forward by inertia (i.e, a tensile force applied to the webbing) is transmitted to the torsion bar via the spool as a twisting force. When the twisting force exceeds a given value, twisting plastic-deformation occurs in the torsion bar. As a result, the locked spool gradually rotates, and the webbing is drawn out while being applied with a predetermined tensile force.
When the torsion bar twists to deform, a torque is applied to the connection part of the spool and the torsion bar in the twisting direction (peripheral direction). A conventional torsion bar used for transmitting the torque is shown in FIG. 7.
FIG. 7(A) is a perspective view of the torsion bar used in the known seat belt retractor. FIG. 7(B) is an expanded front view of a connection part of the torsion bar shown in FIG. 7(A).
In FIG. 7(A), a torsion bar 100 includes a cylindrical bar 102. Connection parts 104 and 106 individually having hexagonal peripheries are formed toward the ends of the bar 102. As shown in the drawing, the left connection part 104 is connected with a reel, and the right connection part 106 is connected with a reel-rotation locking structure. The torsion bar 100 is generally made of a steel wire (according to, for example, JIS Standard S15C) by forging. In this case, the diameters of a base material and a twisting part of the torsion bar 100 (shown by symbol X in FIG. 7(B)) are made the same so that the strength of the torsion bar equals the strength of the base material.
However, since the connection parts 104 and 106 of the torsion bar 100 are hexagonal, apexes 104a and 106a are in line contact along a line parallel to the longitudinal axis of the bar 100 with the reel and parts of the reel-rotation locking structure, respectively. Therefore, when a large twisting force is applied to the torsion bar 100, the apexes 104a and 106a in line contact are likely to be scraped away and to become smooth. Particularly, when the reel, a locking base, or the like, which are members at the concavity side to be coupled (i.e. the side configured to match connection part) with the connection parts, are made of a soft material, such as aluminum. In these situations the concavities of the reel and the like are likely to be scraped away by the twist of the torsion bar.
Another seat belt retractor is disclosed in, for example, Japanese Patent No. 2816332, in which the connection part of the torsion bar is formed in spline so that each convex part of the spline portion is in plane contact. Torsion bars having the spline portions are also disclosed in, for example, Japanese Patent No. 2887120, U.S. Pat. No. 5,899,402, and UK Patent No. GB2314535A. With these arrangements, the connection parts of the torsion bars are not likely to be scraped away and become smooth at the convexities even when the torsion bar twists to deform.
However, most suitable particular spline shapes are not described at all in the above publications.
On the other hand, JIS Standard (B1603) specifies a spline shape, as shown in FIG. 8. According to JIS Standard, the shape shown in the drawing is formed so that a ratio D1/D2 of a tip-circle diameter D1 to a root-circle diameter D2 exceeds 1.3.
However, the ratio D1/D2 according to JIS Standard is excessively large; therefore, the convexities become sharp. The convexities having that sharpness are difficult to form by forging in a simplified process.
Accordingly, an object of the present invention is to provide a seat belt retractor with a torsion bar which can receive a large torque transmitted and can be easily formed by forging.
In order to overcome the aforementioned disadvantages, a seat belt retractor according to the present invention comprises a reel for spooling a seat belt (webbing); a locking mechanism or structure for locking the rotation of the reel; and a torsion bar non-rotatably connected to the reel at one end of the torsion bar and non-rotatably connected to the structure for locking the rotation of the reel at the other end of the torsion bar. The ends (connection parts) of the torsion bar are torx-shaped having a plurality of concavities and convexities formed in a wave shape, and concavities formed in the reel and the structure for locking the rotation of the reel with which the reel and the structure for locking the rotation of the reel are coupled, respectively. A ratio R1/R2 of the distance between the tops of the convexities formed in the wave-shape and the axial center of the torsion bar (radius R1) to the distance between the bottoms of the concavities and the axial center of the torsion bar (radius R2) is set to 1.2 to 1.3.
Since the concavities of the connection parts of the torsion bar and the structure for locking the rotation of the reel are formed in a torx-shape, a torque twisting the torsion bar is transmitted via a plane contact. Therefore, the transmissible torque can be increased compared with the known seat belt retractor having connection parts formed in a hexagon. Moreover, the connection parts can be formed by forging more easily than those having the torx or spline shape specified according to JIS Standard by setting the value R1/R2 to 1.2 to 1.3.
The above forming by forging includes irregular-shape forming by applying a pressure to a bar at the ends thereof. In the torx-shape shown in FIG. 5, the difference in diameter from the bar (see thickness xcex1 shown in FIG. 5) can be set larger than in a hexagonal shape. Therefore, the ends of the torsion bar can be reliably and easily formed.
In the seat belt retractor according to the present invention, it is particularly advantageous to make a material (such as a locking base) to be used for the structure for locking the rotation of the reel, which comes in direct contact with the torsion bar, of aluminum, zinc, or magnesium.
A torque-transmitting part is likely to be scraped and become smooth, and the concavities and convexities thereof are likely to be removed when the torque-transmitting part is made of the above material which is softer than steel or cast iron. Therefore, it is more advantageous to use the material made of aluminum, zinc, or magnesium and formed in a torx-shape.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed.